The soybean, Glycine max (L.) Merril, is one of the major economic crops grown worldwide as a primary source of vegetable oil and protein (Sinclair and Backman, Compendium of Soybean Diseases, 3rd Ed. APS Press, St. Paul, Minn., p. 106. (1989)). The growing demand for low cholesterol and high fiber diets has also increased soybean's importance as a health food.
Soybean yields in the United States are reduced each year by diseases. High yields per hectare are critical to a farmer's profit margin, especially during periods of low prices for soybean. The financial loss caused by soybean diseases is important to rural economies and to the economies of allied industries in urban areas. The effects of these losses are eventually felt throughout the soybean market worldwide. Estimates of loss due to disease in the United States and Ontario vary from year to year and by disease. From 1999 to 2002 soybean yield loss estimates were in the range of 8 million metric tons to 10 million metric tons in the United States and 90,000 to 166,000 metric tons in Ontario (Wrather et al., Online. Plant Health Progress doi:10:1094PHP-2003-0325-01-RV).
Asian Soybean Rust (herein referred to as ASR) has been reported in the Eastern and Western Hemispheres. In the Eastern Hemisphere, ASR has been reported in Australia, China, India, Japan, Taiwan and Thailand. In the Western Hemisphere, ASR has been observed in Brazil, Columbia, Costa Rica and Puerto Rico. ASR can be a devastating disease, causing yield losses of up to 70 to 80% as reported in some fields in Taiwan. Plants that are heavily infected have fewer pods and smaller seeds that are of poor quality (Frederick et al., Mycology 92: 217-227 (2002)). ASR was first observed in the United States in Hawaii in 1994. ASR was later introduced into the continental United States in the fall of 2004, presumably as a consequence of tropical storm activity. Model predictions indicated that ASR had been widely dispersed throughout the southeastern United States, and subsequent field and laboratory observations confirmed this distribution.
Two species of fungi, Phakopsora pachyrhizi Sydow and Phakopsora meibomiae (Arthur) Arthur, cause ASR. Unlike other rusts, P. pachyrhizi and P. meibomiae infect an unusually broad range of plant species. P. pachyrhizi is known to naturally infect 31 species in 17 genera of legumes and 60 species in 26 other genera have been infected under controlled conditions. P. meibomiae naturally infects 42 species in 19 genera of legumes, and 18 additional species in 12 other genera have been artificially infected. Twenty-four plant species in 19 genera are hosts for both species (Frederick et al., Mycology 92: 217-227 (2002)).
Soybean plants resistant to ASR have been identified. Four dominant, independently inherited race-specific QTL for resistance to P. pachyrhizi, herein designated ASR resistance locus 1, ASR resistance locus 2, ASR resistance locus 3, and ASR resistance locus 4, have been identified in PI 200492, PI 230970, PI 462312 (Ankur), and PI 459025B, respectively. These lines, as well as seven others, are suspected of containing QTL for ASR resistance. PI 239871A and PI 239871B (G. soja), PI 230971 and PI 459024B, and the cultivars Taita Kaohsiung-5, Tainung-4, and Wayne have been used as differentials to identify nine races at the Asian Vegetable Research and Development Center, in Taiwan. The predominant race was compatible with three or more of the differentials, indicating that some races already possess multiple virulence factors to known and suspected genes for resistance. Resistance also occurs among the wild Glycine spp. from Australia. Rate-reducing resistance has also been demonstrated. However, it is difficult to evaluate this type of resistance because the rate of rust development is dependent on soybean development and maturity (Sinclair et al., eds., Soybean rust workshop. College of Agricultural, Consumer, and Environmental Sciences. Natl. Soybean Res. Lab. Publ. 1 (1996)).
Evaluating plants that could potentially contain QTL conferring resistance to ASR can be time consuming and require large amounts of biologically contained space. Culturing P. pachyrhizi requires the use of an approved biological containment hood. In addition, greenhouses and growth chambers used to grow plants for ASR resistance testing will have to be constructed in a manner that prevents the accidental release of the organism, especially in locations in which the organism has still not yet been observed. Different cultures of P. pachyrhizi may possess different virulence factors. Over time, new strains of P. pachyrhizi may be introduced into the United States. Therefore, any breeding program designed to breed resistance into soybean against ASR will need to be able to respond rapidly to changes in the P. pachyrhizi population. Also, breeding for soybean crops used in other geographic locations will require selecting resistance to the specific strains that affect those regions, in addition to providing those agronomic characteristics that are preferred by these farmers in that region. Therefore, there is a great need for a rapid, time and cost efficient high throughput method for screening germplasm resistant to ASR. This method must not only provide speed and efficiency, but must also be able to be performed with a minimal amount of space, allowing for the screening of many samples at one time.
The present invention provides a method for screening and selecting a soybean plant comprising QTL for disease resistance.